About the Dilgar sun going nova. I have no evidence for this. But what if the reason the Dilgar turned to conquest was because they knew that their sun was going nova? The Dilgar seemed to have been highly advanced technologically, and it is not beyond the bounds of possibility that they knew their sun was going to go. And the Dilgar, being an aggressive, war-like people, it probably would not occur to them to ask the League of Non-Aligned Worlds if they could move onto a vacant planet. And who would want them as neighbours anyway?